The present invention relates to amplifiers in general, and more particularly to amplifiers which employ pulse duration modulation.
Although the amplifier circuit including the invention will be described in the context of a modulator circuit for an RF transmitter, it should be understood that the invention is not limited to use in this application. Rather, the invention may be used for signal amplifying purposes in general, particularly for those uses requiring power amplification such as hi-fidelity audio systems, public address systems, etc.
In the design of amplifier circuits, the output stage is generally the most critical since it must operate at the highest power level over the entire range of frequencies to be amplified. This is particularly true if the output devices are to be semiconductor devices. The present stage of development of high-powered semiconductor devices is such that the lower cost off-the-shelf devices do not have the desired combination of power output and frequency response. It has been recognized that the inadequate power handling capability of these devices can be circumvented to some extent by connecting a large number of the devices in parallel and driving them with the same signal so that the total power gain is adequate. This approach is exemplified by the patents to Holmes, U.S. Pat. No. 3,348,151; Slenker et al., U.S. Pat. No. 3,022,465; and Schmitt, U.S. Pat. No. 3,443,239. Often, however, special high-cost semiconductor devices must be used to provide the frequency response at the desired power level. This is the case in pulse duration modulation (PDM) amplifiers, wherein not only is the output stage required to respond to the range of signals to be amplified, but must also respond to the switching frequency of the PDM amplifier.
PDM amplifiers have been found to be particularly useful as power amplifiers for modulation stages of RF transmitters. In AM radio broadcasting, Class C or Class D radio frequency modulation stages are commonly employed in order to achieve high efficiency of operation. These RF modulation stages do operate quite efficiently (on the order of 75% efficiency), but also inherently require a large amount of modulation power. For example, a 5 kw RF modulator would require nearly 2.5 kw of audio modulating signal power. Obviously, a high efficiency audio amplifier must also be provided in a system of this sort if a net gain in operation efficiency is to result. Pulse duration modulation (PDM) amplifiers have effectively been used for this purpose since they inherently operate at high levels of efficiency.
System employing PDM amplifier stages are disclosed in the patents to Swanson, U.S. Pat. No. 3,506,920; Heising, U.S. Pat. No. 1,655,543; Bruene et al., U.S. Pat. No. 3,413,570; and Kahn, U.S. Pat. No. 3,648,186.
One problem with these PDM amplifiers arises from the need to use as high a switching frequency in the amplifier as possible in order to preserve high fidelity reproduction of the input signals. Although low fidelity reproduction can be provided with switching speeds which are only two or three times greater than the highest frequency which is to be amplified, in AM broadcasting and other high fidelity applications it has been found that the switching speed should be at least seven times that of the highest frequency of the audio signal in order for the stringent fidelity requirements thereof to be met. Thus, in prior art PDM systems such as Swanson U.S. Pat. No. 3,506,920 the switching speed has been in excess of 70 kHz in order to amplify an audio signal having a bandwidth of 10 kHz. The use of even higher switching frequencies would be desirable from a system performance standpoint, but the implementation of higher frequency switching has previously been impractical due to the additional cost which would be incurred in attempting to provide switching devices capable of operating effectively at these frequencies. Moreover, the off-the-shelf power transistors which would preferably be used in these systems presently only have design operating ranges of approximately 20 to 40 kHz, while currently available power SCR's have a design operating frequency of around 10 kHz. Thus, the devices could not even be employed in systems operating at 70 kHz.
An added problem involves the need for a filter at the output of the PDM amplifier to recover the amplified audio signal from the pulse duration modulated signal. This filter must be capable of removing the frequency component corresponding to the switching signal without significantly affecting the frequency spectrum of the audio component. In prior art systems as mentioned above this meant that the filter would have to provide a very high level of attenuation of a 70 kHz signal while being essentially transparent to a 10 kHz signal. To provide this high degree of filtering, it was necessary in the prior art to use a plural-section filter, the design of which was complicated and rather expensive. The inclusion of such a filter, however, introduced phase shift in the audio signal which caused distortion and overshoot thereof.
This was particularly troublesome in broadcasting applications since many radio stations increase the transmitter modulation level to the point of clipping in order to transmit as "loud sounding" a signal as possible. The level at which this clipping would occur would be set to correspond to the maximum modulation level permitted by the FCC. Since the filter at the output of the PDM amplifier introduced distortion into the square wave signal which resulted from this clipping action, it was possible for the square wave signal to overshoot the clipping level, and thus the maximum modulation level. If FCC modulation restrictions were not to be exceeded, then, it was necessary to reduce the gain of the audio signal below the level which would otherwise be permissible if the distortion occassioned by the PDM output filter were reduced or eliminated.
One manner in which a reduction in distortion could be accomplished is by increasing the switching speed of the PDM amplifier. A reduced amount of filtering would then be required, leading to a corresponding reduction in the distortion introduced thereby. As stated previously, however, an increase in the switching speed has not been feasible in the past.